Russian President Vladimir Putin on Sunday called United States President Trump to thank him after the Central Intelligence Agency helped thwart a purported attack by the Islamic State terrorist group in the northern Russian city of St Petersburg, Reuters reported.

The White House and the Kremlin, however, did not disclose details about the plot, but said the attack, planned for Sunday, “could have killed large numbers of people”. The attackers had reportedly decided to strike crowded sites, including the famous Kazan Cathedral, in the city, The New York Times reported.

The Federal Security Service of Russia on December 15 had announced that it had arrested seven suspected members of a terrorist cell in the city, but did not mention any role the CIA played.

Fourteen people died in an explosion in the city’s Sennaya Ploshchad metro station in April. The attack was carried out by a suicide bomber born in Kyrgyzstan.